Contouring is an important part of radiation therapy planning (RTP), wherein treatment plans are custom-designed for each patient's anatomy. Contours are often obtained in response to user input, wherein a user traces the object boundary on the image using a computer workstation's mouse and screen cursor. However, it should also be noted that contours can also be obtained via automated processes such as auto-thresholding programs and/or auto-segmentation programs.
FIG. 1 depicts an exemplary GUI 100 through which a user can view and manipulate medical images. The GUI 100 includes frame 102 corresponding to the transverse (T) viewing plane, frame 104 corresponding to the coronal (C) viewing plane, and frame 106 corresponding to the sagittal (S) viewing plane. Within frame 102, an image slice of a patient that resides in a T plane can be viewed. Within frame 104, an image slice of a patient that resides in a C plane can be viewed. Within frame 106, an image slice of a patient that resides in an S plane can be viewed. Using well-known techniques, users can navigate from slice-to-slice and viewing plane-to-viewing plane within GUI 100 for a given set of image slices. It can also be noted that the upper right hand frame of GUI 100 depicts a 3D graphics rendering of the contoured objects.
FIG. 2(a) illustrates an exemplary patient coordinate system with respect to a radiotherapy treatment machine that is consistent with the patient coordinate system defined by the IEC 61217 Standard for Radiotherapy Equipment. As can be seen, the patient coordinate system is a right-hand coordinate system such that if a supine patient is lying on a treatment couch with his/her head toward the gantry, the positive x-axis points in the direction of the patient's left side, the positive y-axis points in the direction of the patient's head, and the positive z-axis points straight up from the patient's belly. The origin of this coordinate system can be offset to the origin of the image data under study.
FIG. 2(b) defines the T/S/C viewing planes with respect to the patient coordinate system of FIG. 2(a). As is understood, a plane in the T viewing plane (the xz-viewing plane) will have a constant value for y, a plane in the S viewing plane (the yz-viewing plane) will have a constant value for x, and a plane in the C viewing plane (the xy-viewing plane) will have a constant value for z.
Returning to the example of FIG. 1, the image data within GUI 100 depicts a patient's prostate 110, bladder 112, and rectum 114. As indicated above, an important part of RTP is the accurate contouring of regions of interest such as these.
Current RTP software typically limits contour drawing by the user through GUI 100 to T views (views which are perpendicular to the patient's long axis) as the T images usually have the highest spatial resolution, the T images are the standard representation of anatomy in the medical literature, and the T contours are presently the only format defined in the DICOM standard. The two other canonical views—the S and C views—can then be reconstructed from the columns and rows, respectively, of the T images.
When generating 3D surfaces from image slices, conventional software programs known to the inventor herein allow the user to define multiple T contours for a region of interest within an image for a plurality of different T image slices. Thereafter, the software program is used to linearly interpolate through the different T contours to generate a 3D surface for the region of interest. However, the inventor herein notes that it is often the case that a plane other than a T plane (e.g., planes within the S and/or C viewing planes) will often more clearly depict the region of interest than does the T plane. Therefore, the inventor herein believes there is a need in the art for a robust 3D contouring algorithm that allows the user to define input contours in any viewing plane (including S and C viewing planes) to generate a 3D surface for a region of interest and/or generate a new contour for the region of interest.
Further still, the inventor herein believes that conventional 3D surface generation techniques, particularly techniques for generating variational implicit surfaces, require unacceptably long computational times. As such, the inventor herein believes that a need exists in the art for a more efficient method to operate on contours in three dimensions.
Toward these ends, according to one aspect of an embodiment of the invention, disclosed herein is a contouring technique that increases the efficiency of 3D contouring operations by reducing the number of data points needed to represent a contour prior to feeding those data points to a 3D contouring algorithm, wherein the 3D contouring algorithm operates to generate a 3D surface such as a variational implicit surface or process the reduced data points to generate a new contour in a new plane via an interpolation technique such as B-spline interpolation. The data points that are retained for further processing are preferably a plurality of shape-salient points for the contour. In accordance with one embodiment, computed curvature values for the data points are used as the criteria by which to judge which points are shape-salient. In accordance with another embodiment, computed scalar second derivative values are used as the criteria by which to judge which points are shape-salient. In accordance with yet another embodiment, the DeBoor equal energy theorem is used as the criteria by which to judge which points are shape-salient.
According to another aspect of an embodiment of the invention, disclosed herein is a contouring technique that operates on a plurality of data points, wherein the data points define a plurality of contours corresponding to a region of interest within a patient, each contour being defined by a plurality of the data points and having a corresponding plane, wherein the plurality of data points are reduced as described above and processed to find the reduced data points that intersect a new plane, and wherein B-spline interpolation is used to interpolate through the points of intersection to generate a new contour in the new plane. This embodiment can operate on a plurality of contours drawn by a user in the S and/or C viewing planes to generate a T contour in a desired T plane. The point reduction operation performed prior to the B-spline interpolation improves the efficiency of the B-spline interpolation operation.
While various advantages and features of several embodiments of the invention have been discussed above, a greater understanding of the invention including a fuller description of its other advantages and features may be attained by referring to the drawings and the detailed description of the preferred embodiment which follow.